I don't think this is possible unless you have access to the site's Adsense dashboard. However, you can study how the target keywords are performing in Adwords to the site's ranking and you have yourself an approximation.

Send an email to the site owner suggesting that you want to buy the website, when they say how much then make a silly deal on the terms they disclose how much the site has earnt in the last 3 months

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Man you are very clever!

Ok just do a math. Analyse how many visitors that site getting. You can get some payed and free tools online to know this.
Take list of KW's that site using put that on adwords tool get the CPC of all & get average CPC out of that. Assume that site getting 3% adsense conversion.
Calculate with avg CPC and visitors. You will get approximate revenue calculation.

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